Two distinct species of caprine arthritis encephalitis virus (CAEV) tat cDNAs were isolated early after infection. Sequence analyses predicted that one cDNA (pCEV/e1) represented a polycistronic transcript that encodes Tat and Rev as well as an N-terminally truncated transmembrane protein and a protein. designated X, whose function is unknown; whereas the other cDNA (pCEV/f1) encodes Tat and the env gene products, pCEV/e1 trans-activated a CAEV long terminal repeat (LTR)-chloramphenicol acetyltransferase (CAT) reporter gene. The target sequences for Tat within the CAEV LTR were localized to the U3 region which, when placed in either orientation upstream of heterologous promoters, was able to confer responsiveness to Tat. The Tat protein of equine infectious anemia virus (EIAV) was synthesized in Escherichia coli with yields of up to 20 mg per 10(11) cells. In eukaryotic cells. the recombinant Tat potently transactivated the EIAV LTR and localized to the cytoplasm. The target (RRE) for EIAV Rev was found to differ from those of other lentiviruses by lacking strong secondary structure and a singe location within the env intron. Lymphoproliferative disease virus infection does not induce overt clinical manifestations; thus, early detection is essential for preventing disease that can decimate flocks. A simple, rapid, sensitive and highly specific assay using the polymerase chain reaction was developed and is capable of providing differential diagnosis of the disease soon after infection.